1. Field of the Invention
The present invention relates to a method and apparatus for performing buffer status reporting (BSR), and more particularly, to a method and apparatus for indicating a format of a BSR MAC control element by a MAC PDU sub-header corresponding to the BSR MAC control element.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE, MAC provides transmission services via a plurality of logic channels. To manage uplink resources, when a user equipment (UE) performs uplink (UL) transmission, the network can divide or map logic channels of the UE into at most four groups based on priorities, types, and so on. The priorities are corresponding to values 1 to 8, and assigned by a higher layer, the radio resource control layer. Besides, the 3rd Generation Partnership Project, 3GPP, introduces a Buffer Status Reporting (BSR) procedure, which is used to provide the serving NB (or enhanced NB) with information about the amount of data in UL buffers of the UE. In the BSR procedure, the UE uses a MAC PDU (Protocol Data Unit) to carry a BSR MAC control element with information about the amount of data in the UL buffers to the network. Accordingly, the network can determine the total amount of data available across one or all logical channel groups.
In a current specification, there are three types of BSR procedures for different triggering events, a regular BSR, a periodic BSR and a padding BSR. The regular BSR is triggered when UL data arrives at the UE transmission buffer and the UL data belongs to a logical channel with higher priority than those for which data already existed in the UE transmission buffer, or is triggered when a serving cell change occurs. The periodic BSR is triggered when a periodic BSR timer expires. The padding BSR is triggered when UL resources are allocated and an amount of padding bits is equal to or greater than the size of the BSR MAC control element.
Besides, according to different requirements, the BSR MAC control elements can be categorized into two formats: short and long. A short BSR MAC control element is 1-byte long, and has 8 bits, where the former 2 bits indicate one logic channel group which buffer status is being reported, and the remaining six bits indicate the amount of buffered data available across the logic channel group. A long BSR MAC control element is 3-byte long, and utilized for reporting data amount of the UL buffers of all the logic channel groups.
On the other hand, a MAC PDU header consists of one or more sub-headers, each sub-header corresponding to either a MAC Service Data Unit (SDU), a MAC control element or padding carried by the MAC PDU. The MAC PDU sub-headers have the same order as the corresponding MAC SDUs, MAC Control elements and padding. A MAC PDU sub-header corresponding to a MAC control element generally consists of two Reserved bits, an 1-bit Extension field and a 5-bit logic channel indentify (LCID) field. In the current specification, the format of the BSR MAC control element can be identified by LCID values, e.g. “11101” and “11110” are used for indicating the short BSR MAC control element and the long BSR MAC control element, respectively. Detailed description of the MAC PDU sub-headers can be found in related protocol specifications, and not narrated herein.
For the regular and periodic BSR procedure, the format of the BSR MAC control element is determined by the UE according to whether there is more than one logic channel group with uplink buffered data in the UE when the BSR is transmitted. If there is only one logic channel group with uplink buffered data, the short BSR MAC control element is reported. Conversely, if there are more than one logic channel groups with uplink buffered data, the long BSR MAC control element is reported.
For the padding BSR procedure, the BSR control element format is determined according to the number of padding bits remaining in the MAC PDU. If the number of padding bits is equal to or larger than the size of the short BSR MAC control element but smaller than the size of the long BSR MAC control element, the short BSR MAC control element is used, for reporting data amount in the uplink buffer of a highest priority logic channel group. Conversely, if the number of padding bits is equal to or larger than the size of the long BSR MAC control element, the long BSR MAC control element is used, for reporting data amount in the uplink buffer of all the logic channel groups.
In this case, when there are more than one logic channel groups with uplink buffered data but the number of padding bits of the MAC PDU cannot accommodate the long BSR MAC control element, based on the related specifications, the UE would still report the short BSR MAC control element for increasing system scheduling efficiency. However, since the prior art does not disclose how to indicate the short BSR MAC control element transmitted by the UE is triggered by the regular and periodic BSR procedure or by the padding BSR procedure, the network is unable to determine whether there is only one logic channel group with UL buffered data in the UE. It is possible that a wrong decision, such as providing an incorrect uplink grant for the UE, may be made by the network.